Acid gold plating



United States Patent Office 3,149,557 Patented Sept. 15, 1964 3,149,057 ACID GOLD PLATING Edward A. Parker, Cranston, and James A. Powers, East Providence, R1,, assignors to Technic, inn, Providence, R.I., a corporation of Rhode island No Drawing. Filed Apr. 27, 1959, Ser. No. 808,913 11 Claims. (Cl. 204-46) The present invention relates to the electrolytic deposition of gold on surfaces and more particularly to the electrodeposition of gold and gold alloys from solutions of the same onto objects to develop a gold or gold alloy appearance.

It has been customary for many years in the electroplating of gold and gold alloys to use a bath containing an alkali metal 'aurocyanide, with or without other soluble metal salts. In addition to the above compounds, certain other additives have been used to increase the conductivity of the bath; to increase throwing power; to maintain color; or to provide a buffered pH region in which the bath was to operate. A few examples of these additives are free alkali metal cyanide, alkali metal hydroxide, alkali metal carbonate or bicarbonate, and, almost always, a di-basic, or tri-basic alkali metal phosphate. In all instances, however, the pH of the aurocyan-ide bath in plating operations is never reduced below pH=6, and is most usually maintained in the pH range of 8 to 12.

Gold baths of this conventional type containing free alkali cyanide are never allowed to operate on the acid side, because of the formation and elimination from the bath of the extremely toxic hydrogen cyanide gas. An additional chemical reason for maintaining alkaline baths resides in the fact that where an alkali metal aurocyanide solution is contacted with a strong mineral acid, one cyanide radical is displaced from the aurocyanide complex anion and there separates from the solution insoluble aurous cyanide.

Such 24 karat baths must be operated hot and are usually limited to rather low current densities. It is also diflicult to obtain bright deposits in heavy thicknesses, i.e. greater than .000050". It, therefore, has been common practice in the prior art to add to such a bath certain soluble metal salts which are co-deposited with the gold in order to impart certain qualities to the plate such as color, hardness, wear resistance, etc.

We have found that the addition of the cobalt (II) chelate of ethylenediamine tetra-acetic acid, or other chelating agent giving with cobalt (II) a chelate having a stability constant of the same order of magnitude, to an alkali metal aurocyanide at a relatively low pH will produce a bright, smooth electrodeposit of gold which is better than 99.8% pure. The amount of cobalt chelate employed in the solution of gold to effect this result may vary from 1 gram per liter up to 50 grams per liter.

However, we prefer to operate in the region of 20-30 grams per liter. Heavy deposits may also be obtained which are brilliant in character. By the addition of this compound the speed of deposition for bright plating is immensely increased so that current densities of 60 amperes per square foot are possible. It has also been found that such 24 karat plates are of smaller grain size, increased hardness, less porous, and have better wear resistance than similar plates obtained from a conventional 24 karat bath.

It appears that the effect of having present in the solution a cobalt salt holding the cobalt (II) in such stable form that it does not plate out is that the cobalt inhibits the growth of coarse crystallites of gold in the plated surface. It thereby permits the development of bright plate, pure, at low voltage, high current density over a wide range to assist in obtaining full coverage overall parts of the area.

Gold alloy plates may also be obtained from this bath by incorporating into the bath certain metal salts, complexes, or chelates, because such other metals plate out with the gold. Among such compounds may be men tioned antimony tartrate, nickel or cadmium ethylenediamine tetra-acetic acid. Similar compounds of indium, zinc, copper, iron may be used to obtain alloy plates.

It is, therefore, a primary object of the present invention to provide an aqueous acid solution from which gold may be electrodeposited in which the harmful consequences of the formation of free hydrogen cyanide are eliminated.

It is another object of this invention to provide a bath for the deposition of substantially 24 karat gold electroplate which is characterized by higher hardness, greater wear resistance, and increased speed of plating than that obtained in a conventional 24 karat bath.

It is another object of the present invention to provide a solution from which gold and gold alloys may be electrodeposited in a clean, smooth fashion to give a gold or gold alloy plate which is of high reflectivity and will provide the mirror image of an object placed before it.

It is a further object of the present invention to provide a solution from which gold and gold alloys may be electrodeposited in which the harmful effects of alkali and alkali producing materials are avoided.

The present invention, accordingly, comprises a bath from which gold may be electrolytically deposited, the bath essentially consisting of an aqueous solution of an alkali metal aurocyanide, with or without additional a1 loying salts, the cobalt chelate of ethylenediarnine tetraacetic acid, or a cobalt che-late of a chelating agent having a stability constant of about the same order of magnitude, and at least one buffer material to maintain the pH between 3.0 and 4.5 during the deposition of the gold from the solution.

The maintenance of the pH value between 3.0 and 4.5 is most easily accomplished by incorporating into the bath salts of moderately strong organic acids whose acid dissociation constants are of such a nature that they serve as buffers in restricting the pH of the solution to limits as defined during the electro-deposition of the gold or its alloy. Among these substances may be listed, as representative, certain organic acids which have at least one ionization constant of such a value as to create a buffer region in the specified pH range, when properly neutralized with the appropriate amount of base.

In general, the acids most useful are the soluble aliphatic acids containing 2 to 8 C atoms, and also may carry hydroxyl groups in functional relationship to the carboxylic groups. Typical useful acids are citric, citraconic, itaconic, lactic, gluconic, glutaric and glycollic.

This list, while not all inclusive of the substances which may be used, is indicative of the general types of compounds which have been found to be useful. It is also possible to use mixtures of the compounds, provided the pH of the final gold bath lies in the range of 3.0 to 4.5.

The electrolytic deposition can be carried out using insoluble anodes such as platinum, gold, stainless steel or carbon, and replenishment of the gold and its alloying components can be accomplished during plating to maintain these materials at their original concentrations as the bath is used. The ratio of anode surface area to cathode surface area should be most preferably not less than 1:1. The temperature of the bath should be held within the limits of 55 to F. during the electrodeposition. It has been found that mechanical agitation of the bath, such as with a stirrer, serves to facilitate clean, smooth deposition, and at the same time permits the use of higher current densities without smutting the plated surface. The electrical tension between the anodes and the wares is held between 2 and 4 volts.

The following examples illustrate typical composition for use in plating operations in accordance with this invention:

Example I Into an amount of water suificient to form 1 liter of solution is dissolved:

12 grams of pure gold in the form of potassium gold (I) cyanide t 105 grams of citric acid 56 grams of potassium hydroxide 12.6 milliliters con. phosphoric acid 1 gram Co as cobalt dipotassium of ethylenediamine tetra-acetic acid This solution may be utilized for the electrodeposition of gold in the usual manner with a current density of 5 amperes per square foot (a.s.f.).

Example [1 Into an amount of water sufiicient to form 1 liter of solution is dissolved:

12 grams of pure gold in the form of potassium gold (I) cyanide 125 grams of primary potassium citrate 0.2 gram of antimony as antimony tartrate 3.0 grams Co as dipotassium cobalt ethylenediamine tetra-acetate Example 111 Into an amount of water suilicient to form 1 liter of solution is dissolved:

ethylenediamine The pH is then adjusted with either phosphoric acid or potassium hydroxide to a value near 4.0, but no lower than 3.0 nor higher than 4.5.

Example i V Into an amount of water suflicient to form 1 liter of solution is dissolved:

6 grams of pure gold in the form of potassium gold (I) cyanide 28 grams of glacial acetic acid 40 grams of potassium acetate 1 gram of cobalt as potassium cobalt ethylene diamine tetra-acetic acid ExampIe V Into an amount of water sufficient to form 1 liter of solution is dissolved:

8 grams of pure gold in the form of potassium gold (I) cyanide 50 grams of monopotassium phosphate 50 grams of citric acid 21 grams of potassium hydroxide 3 grams of cobalt as dipotassium cobalt ethylene diamine tetra-acetic acid In each case the pH should be finally adjusted with phosphoric acid or potassium hydroxide to a value of 4.0 but no lower than 3.0 or greater than 4.2.

Copper panels plated at 10 amperes per square foot at 75 F. with vigorous agitation for 20 minutes gave smooth brilliant gold plates with the appearance of 24K .asgosr gold. Plate thickness was 0.0002 inch indicating a current elliciency of 40 percent.

Barrel plating of small items is readily consummated in the type of solution illustrated in Examples I-V with a still further improvement in appearance and uniformity even at current densities as low as 1-5 amperes per square foot. The formulation given in Example V will give excellent results. The gold content may be reduced to onehalf this value to reduce losses in dragout.

It will be evident from the specific examples given that the preferre gold plating formula of this invention includes a very substantial molar excess of the organic acid over the amount of gold present. Generally it may be said that at least two moles of the acid or mixture of acids being used per mole of gold is a preferred range. Within said range the gold is held in complex form, hydrogen cyanide is not released, optimum pH in the range of 3-5 is achieved and gold plate of mirror-bright finish is consistently produced.

Typical ethylenediaminetriacetic acid compounds which are useful chelating agents for holding the cobalt in the form of a complex of the requisite of stability to accomplish the results of the invention are ethylenediaminetetraacetic acid, hydroxyethyl ethylenediaminetriacetic acid, hydroxypropyl ethylenediaminetriacetic acid, ethylenediaminediacetic acid and the like. It seems that the essential element of structure in the compound is a spaced acetic acid moiety which will form a cobalt com plex of the desired level of stability.

Though the invention has been described in terms of only a few specific examples, variance thereof may be practiced Without departing from its spirit or scope.

What is claimed is: Y

1. In a bright gold plating bath consisting essentially of a Water solution of gold cyanide, the improvement which comprises incorporating and maintaining in said solution a cobaltous chelate of an ethylenediamine acetic acid compound, the cobaltous chelate having a stability such that it is not electrodeposited with the gold under plating conditions, the solution being buffered at a pH level in the range from about 3 to about 5, the chelate being such that cobalt is not deposited under plating conditions.

2. An electroplating composition suitable for the deposition of gold from an acid medium consisting essentially of 1-50 parts of gold by weight as potassium gold cyanide, and up to 25 parts of cobalt by weight as potassium cobaltous chelate of ethylencdiaminetetraacetic acid, the Weight ratio of gold to cobalt being at least about 2:1, in a buffering medium including a potassium phosphate, citric acid and potassium citrate, the said composition when dissolved in water forming a solution at plating concentration of 5-25 grams of gold per liter of solution for the deposition of gold having a pH in the range from about 3 to about 5, said cobalt chelate being such that cobalt is not plated from the solution under plating conditions.

3. A composition suitable for formulation of acid gold plating solutions comprising a mixture of potassium gold cyanide and potassium cobaltous ethylenediamine tetraacetate, the ratio of the two being about 1-50 parts of the gold by weight to as much as 25 parts of cobalt by weight. 4. An aqueous gold plating solution in accordance with claim 2 consisting essentially of:

12 parts by weight of pure gold as potassium auro (I) cyanide, 1 part by weight of cobalt as potassium cobaltous chelate of ethylenediamine-tetraacetic acid, and suflicient phosphoric acid, citric acid and potassium hydroxide to maintain the pH of said solution at a level in the range from 3 to 5.

5. An aqueous gold plating solution in accordance with claim 2 consisting essentially of:

12 parts by Weight of pure gold as potassium auro (I) cyanide,

3 parts by weight of cobalt as potassium cobaltous chelate of ethylenediaminetetraacetic acid, 125 parts by weight of potassium citrate,

.2 part by weight of antimony as antimony tartrate.

6. An aqueous gold plating solution in accordance with claim 2 consisting essentially of:

12 parts by weight of pure gold as potassium auro (I) cyanide,

1 part by weight of cobalt as potassium cobaltous chelate of ethylenediaminetetraacetic acid,

2 parts by weight of indium as the indium chelate of ethylenediaminetetraacetic acid, and sufiicient citric acid and potassium hydroxide to maintain the pH in the range from 3 to 4.5.

7. An aqueous gold plating solution in accordance with claim 2 consisting essentially of:

6 parts by weight of pure gold as the potassium auro (I) cyanide, 1 part by weight of cobalt as the potassium cobaltous chelate of ethylenediaminetetraacetic acid, and sufiicient acetic acid and potassium acetate tomaintain the pH of said solution at a level in the rangefrorn 3 to 5.

8. An aqueous gold plating solution in acordance with claim 2 consisting essentially of:

8 parts by weight of pure gold in the form of potassium auro (I) cyanide,

3 parts by weight of cobalt as potassium cobaltous chelate of ethylenediaminetetraacetic acid, and sufficient monopotassium phosphate, citric acid and potassium hydroxide to maintain the pH of said solution at a level in the rangefrom 3 to 4.

9. In the method of electroplating gold from aqueous solutions, the improvement comprising forming a water solution of gold cyanide and maintaining in said solution a minor amount of cobalt of valence II as a chelate of an ethylenediamine acetic acid compound, said solution containing buifering ingredients suitable to maintain the pH of the solution in the range from about 3 to about 5, so that at current densities up to 100 amperes per square foot'a bright gold plate is achieved at temperatures up to 100 F. without deposition of cobalt.

10. In the process of plating gold to obtain bright gold deposits on objects in accordance with claim 9, the improvement comprising formulating said gold as the auro (I) cyanide in an aqueous medium wherein the pH of said medium is maintained in the range from about 3 to about 5, in the presence of organic acids selected from the group consisting of acetic, citric, citratonic, itaconic, glutaric, lactic succinic and gluconic acidsand mixtures thereof, and also in the presence of a cobaltous chelate of a compound selected from the group consisting of ethylenediaminetetraacetic acid, hydroxyethyl ethylenediamine triacetic' acid, hydroxypropylethylenediaminetriacetic acid, and ethylenediaminediacetic'acid.

11. In'the electroplating of gold from aqueous solution in accordance with claim 9, the improvement which comprises maintaining the gold in solution as the auro (I) cyanide at a pH in the range from about 3 to about 5 in the presence of an alloying metal for simultaneous electrodepo'sition with gold, the improvement comprising acetic acid, hydroxyethyl ethylenediamine triacetic acid,

maintaining in the said solution acobaltous chelate selected from the group consisting of ethylenediaminetetrahydroxypropylethylenediaminetriacetic acid, and ethylenediaminediacetic acid.

References Cited in the file of this patent UNITED STATES PATENTS Ostrow et al. Jan. 3, 1961 

1. IN A BIRGHT GOLD PLATING BATH CONSISTING ESSENTIALLY OF A WATER SOLUTION OF GOLD CYANIDE, THE IMPROVEMENT WHICH COMPISES INCORPORATING AND MAINTAINING IN SAID SOLUTION A COBALTOUS CHELATE OF AN ETHYLENEDIAMINE ACETIC ACID COMPOUND, THE COBALTOUS CHELATE HAVING A STABILITY SUCH THAT IT IS NOT ELECTRODEPOSITED WITH THE GOLD UNDER PLATING CONDITIONS, THE SOLUTION BEING BUFFERED AT A PH LEVEL IN THE RANGE FROM ABOUT 3 TO ABOUT 5, THE CHELATE BEING SUCH THAT COBALT IS NOT DEPOSITED UNDER PLATING CONDITIONS. 